NONLINEAR OPTICAL CdSiP2 CRYSTAL AND PRODUCING METHOD AND DEVICES THEREFROM

ABSTRACT

CdSiP 2  crystals with sizes and optical quality suitable for use as nonlinear optical devices are disclosed, as well as NLO devices based thereupon. A method of growing the crystals by directional solidification from a stoichiometric melt is also disclosed. The disclosed NLO crystals have a higher nonlinear coefficient than prior art crystals that can be pumped by solid state lasers, and are particularly useful for frequency shifting 1.06 μm, 1.55 μm, and 2 μm lasers to wavelengths between 2 μm and 10 μm. Due to the high thermal conductivity and low losses of the claimed CdSiP 2  crystals, average output power can exceed 10 W without severe thermal lensing. A 6.45 μm laser source for use as a medical laser scalpel is also disclosed, in which a CdSiP 2  crystal is configured for non-critical phase matching, pumped by a 1064 nm Nd:YAG laser, and temperature-tuned to produce output at 6.45 μm.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/107,876, filed Oct. 23, 2008, incorporated herein by reference in itsentirety for all purposes.

STATEMENT OF GOVERNMENT INTEREST

The present invention was made in conjunction with Government fundingunder contract number FA8650-05-C-5425 awarded by the United States AirForce. The United States Government has certain rights in thisinvention.

FIELD OF THE INVENTION

The invention relates to non-linear optics, and more particularly to theproduction and use of birefringent crystals in nonlinear opticaldevices.

BACKGROUND OF THE INVENTION

Mid-infrared lasers are important for a growing number of applications,such as spectroscopy, remote chemical sensing, laser surgery, andinfrared countermeasures. Typically, laser output at mid-infraredwavelengths is produced by using a nonlinear optical (NLO) crystal toshift the output wavelength of a solid state laser, such as a Nd:YAG(1.06-μm) laser or an erbium-doped fiber laser (1.55-μm), to awavelength in the 2-12 μm range. Oxide-based crystals such as potassiumtitanyl phosphate, KTiOPO₄ (KTP), and periodically-poled lithiumniobate, LiNbO₃ (PPLN) work well at the short end of this spectralrange, but the output power from these materials falls off dramaticallybeyond 4 microns.

The I-III-VI₂ chalcopyrite crystals AgGaS₂ and AgGaSe₂ can be pumped at1.06 μm and 1.55 μm respectively so as to generate output much deeperinto the infrared (up to 12 μm), but they are plagued by low damagethresholds and extremely poor thermal properties that preclude their usefor high average power applications. Crystals of ZnGeP₂, a II-IV-V₂chalcopyrite, have significantly higher nonlinear coefficients and muchbetter thermo-mechanical properties for high power operation, but theymust be pumped by less common sources such as Tm- or Tm,Ho-lasersoperating at wavelengths greater than 1.9 μm. Furthermore, the outputpower and efficiency of 2-micron-laser-pumped ZnGeP₂ optical parametricoscillators (OPOs) is limited by absorption losses at the pumpwavelength.

II-IV-V₂ chalcopyrites like ZnGeP₂ are derived from III-V compoundsemiconductors (like GaP) by ordered substitution of group II (Zn) andgroup IV (Ge) atoms on the group III (Ga) site in the same way thatI-III-VI₂ chalcopyrites like AgGaS₂ and AgGaSe₂ are derived from II-VIcompound semiconductors (like ZnSe). While the I-III-VI₂ compounds offerlow absorption losses, they are usually plagued by scattering losses dueto precipitates which form on cooling as a result of off-stoichiometrycongruent melting and retrograde solubility of the III₂VI₃-rich phase.They tend to have larger band gaps than their II-IV-V₂ counterparts, butare inferior in terms of hardness, thermal conductivity, anisotropicthermal expansion, and thermo-optic coefficients, all of which makesthem prone to thermal lensing and laser-induced damage in both thesurface and the bulk. They also have lower nonlinear coefficients thanthe II-IV-V₂ chalcopyrites.

The II-IV-V₂ compounds, therefore, offer superior properties in almostevery respect. They are generally free of scattering centers, andabsorption losses (at least in the phosphides) can be very low in thecenter of their transparency range. While they can be plagued bydefect-related absorption losses near the band edge when grown from themelt, these losses can often be eliminated by post-growth processing.

Of course, not all II-IV-V₂ compounds are suitable for nonlinear opticalapplications. To be useful for nonlinear optical frequency conversion, acrystal must have sufficient birefringence for phase-matching. Becausethe speed of light (as determined by the refractive index, n) inside acrystal varies as a function of wavelength (a phenomenon known as“dispersion”), the input and output waves will normally remain in phaseover a very short distance (˜10-100 μm in the infrared), termed the“coherence length,” beyond which frequency conversion ceases. Abirefringent crystal exhibits two refractive indices: one for lightpolarized perpendicular to the optic axis (ordinary- or o-polarized) andanother for light polarized parallel to the optic axis (extraordinary-or e-polarized). If the difference between the two refractive indices(i.e., the birefringence, n_(e)−n_(o)) exceeds the difference inrefractive index at the two wavelengths (i.e., the dispersion), then adirection inside the crystal can be chosen such that input and outputwaves of opposite polarization can experience the same refractive index.Under these conditions, the two waves remain in phase (i.e. are“phase-matched”), allowing the frequency conversion process to build toreasonable efficiencies over a long interaction length.

In addition to phase matching, a nonlinear optical crystal must behighly transparent at the input and output wavelengths of interest, andit must have a band edge that is near or below one-half of the shortestwavelength involved in the frequency conversion process, so as to avoidtwo-photon absorption. For down-conversion processes such as opticalparametric oscillation (OPO), optical parametric amplification (OPA),and difference frequency generation (DFG), the input wave represents theshortest wavelength, whereas for up-conversion processes such as secondharmonic generation (SHG) or sum frequency generation (SFG) the shortestwavelength(s) is/are at the output. Pumping with a 1064 nm Nd:YAG solidstate laser, for example, requires a band edge near or below 532 nm.

Finally, even if a candidate compound has adequate nonlinearity,transparency, and phase-matching properties, it will not be possible touse the compound as the basis of an optical device unless a method isfound for growing crystals of sufficient size and optical quality sothat devices with the required crystallographic orientation can befabricated. Oriented crystals with dimensions of at least 3 mm×3 mm×10mm are typically required for practical devices, and the crystals mustbe free of cracks, twins, voids, inclusions, grain-boundaries, and othermacroscopic defects, at least within the aperture and propagating pathof the interacting waves.

Although at the time the present invention was made, CdSiP₂ was known tobe a II-IV-V₂ chalcopyrite compound with at least some propertiesconsistent with use in NLO devices, CdSiP₂ was not generally consideredby those skilled in the art to be a likely candidate for practical NLOdevices. The crystal structure of CdSiP₂ had been reported by S. C.Abrahams and J. L. Bernstein, J. Chem. Phys. Vol. 55, p. 796 (1971),incorporated herein by reference for all purposes. Specifically,Abrahams and Bernstein had reported that CdSiP₂ belongs to thetetragonal space group 42m, that its lattice parameters are respectively5.68 A and 10.431 A for a and c, and Z=4 in each unit cell, and that themass density of CdSiP₂ is 4.70 g/cm³.

The birefringence of tiny 2×2×0.2 mm³ CdSiP₂ crystals grown from amolten tin flux had been measured in the prior art and reported in N.Itoh, T. Fujinaga, and T. Nakau, “Birefringence in CdSiP₂,” Jap. J.Appl. Phys. 17, 951-2, (1978) (incorporated herein by reference for allpurposes) to be −0.045 at 840 nm, but no measurements were made in theinfrared.

Several other optically relevant properties of CdSiP₂ were eitheruncertain or unknown at the time the present invention was made. Forexample, values reported in the prior art for the band gap of CdSiP₂ hadranged from 2.2 eV (563 nm) to 2.45 eV (506 nm), causing it to beunclear if CdSiP₂ would exhibit two-photon absorption if pumped by apulsed 1064 nm laser. The long-wavelength transparency limit of CdSiP₂had not been measured in the prior art. And while tiny samples (up to5×2×1 mm³) had been grown in the prior art by halogen-assisted vaportransport, as reported for example in E. Buehler and J. H. Wernick,Journal of Crystal Growth 8, 324 (1971), incorporated herein byreference for all purposes, the CdSiP₂ crystals produced thereby werefar too small to be used for nonlinear optical devices. In fact, theseprior art crystals were too small to enable measurement of the mostrelevant nonlinear optical properties, including the nonlinearcoefficient, the transparency range, and the phase-matchingcharacteristics. Furthermore, the methods used to grow these prior artcrystals were not scalable to produce crystals having the required sizeand quality.

At the time the present invention was made, it therefore appeareddoubtful to those of average skill in the art that CdSiP₂ crystals ofoptical device size and quality could be grown. Buehler and Wernick hadreported an approximate melting point of 1120° C. for CdSiP₂ and a vaporpressure of 19.4 atm at 1100° C. (which extrapolates to 20.4 atm at1120° C. and 23.9 atm at 1175° C.). The present inventors had evenattempted to grow CdSiP₂ directly from a stoichiometric melt withoutsuccess, finding that the quartz ampoules devitrified and reacted withCdSiP₂, and that the solidified melts were porous and polycrystalline.An equilibrium phase diagram for CdSiP₂ system had not been determinedin the prior art, and it was not known if the compound would meltcongruently, which is a necessary condition for melt growth.

A method was known in the prior art (see U.S. Pat. No. 5,611,856,incorporated herein in its entirety for all purposes) for producingsingle crystals of group II-IV-V₂ compounds of sufficient size andoptical quality for use in NLO devices by pre-synthesizing the compoundmaterial from its constituents and separately melting andre-crystallizing by directional solidification in sealed quartz ampoulesusing a two-zone horizontal transparent furnace. Crystals with meltingpoints as high as 1027° C. and vapor pressures as high as 7 atmosphereshad been grown by this method. However, it did not appear at the time ofthe present invention that this method would tolerate the significantlyhigher temperatures (˜1175° C. in the hot zone) and pressures (˜24 atm)required to produce a CdSiP₂ crystal. In addition, it was not knownwhether the compound itself would melt congruently, and whether thequartz ampoules could be used without devitrifying or reacting with thecompound. Finally, it was not apparent that a crystal of CdSiP₂ would betransparent and phase-matchable for mid-infrared frequency conversion,even if a crystal of sufficient size and quality could somehow be grown.

For all of these reasons, at the time the present invention was madeCdSiP₂ was not generally considered by those skilled in the art to be alikely candidate crystal for practical NLO device applications.

A need exists, therefore, for CdSiP₂ single crystals of sufficient sizeand optical quality to be suitable for use in NLO devices, and for amethod of producing such crystals. A need also exists for NLO devicesthat use such CdSiP₂ crystals to frequency-shift the output of 1.06 μmand 1.55 μm lasers to mid-infrared wavelengths ranging from 2-10 μm, andfor frequency-shifting 2-μm lasers at high efficiencies and outputpowers into the 3-10 μm range. Furthermore, a need exists for a methodof using such crystals of frequency-shifting the output of 1.06 μm, 1.55μm, and 2-μm lasers to mid-infrared wavelengths ranging from 2-10 μm.

SUMMARY OF THE INVENTION

Large, high optical quality CdSiP₂ crystals suitable for use innonlinear optical devices are claimed, as well as a method of producingsuch crystals by directional solidification from a stoichiometric melt.CdSiP₂ crystals of sufficient size and optical quality for use in NLOdevices do not occur in nature, and could not be artificially producedbefore invention of the method claimed herein. The crystals of thepresent invention are therefore compositions of matter with essentialdifferences in their properties as compared to naturally occurring andprior art manufactured crystals, wherein these essential differencesinclude enhanced optical quality and size that enable the claimedcrystals to be used as the basis of practical NLO devices, in contrastwith CdSiP₂ crystals of the prior art that cannot be so used.

The claimed method of producing NLO-compatible CdSiP₂ crystals issimilar to the prior art method of U.S. Pat. No. 5,611,856. However, itwas unexpected at the time the present invention was made that largeingots of the compound CdSiP₂ could be successfully synthesized withoutexplosion of the quartz ampoule due to the high temperatures andpressures required. Furthermore it was unexpected that large, highoptical quality single crystals could be successfully grown inhorizontal, transparent furnaces by directional solidification at therequired temperatures and pressures.

Nonlinear optical devices are also claimed that use the CdSiP₂ crystalsof the present invention to produce a shifted wavelength laser beamhaving a wavelength different from all incident beams. The claimeddevices are capable of efficiently shifting laser wavelengths as shortas 1 μm so as to produce electromagnetic radiation with wavelengths inthe 2-10 μm wavelength range, and with average power levels in excess of10 W, while avoiding severe thermal lensing due to the low absorptionlosses and high thermal conductivity of the claimed CdSiP₂ crystals.

One general aspect of the present invention is a nonlinear opticaldevice comprising a negative uniaxial II-VI-V₂ crystal belonging to thespace point group 42m and having NLO properties, whereby at least oneincident beam of electromagnetic radiation can be directed into saidcrystal so as to generate electromagnetic radiation emerging from saidcrystal that includes at least one output wavelength different from thewavelengths of all incident beams of radiation, and wherein said crystalis a single crystal of CdSiP₂. In various embodiments, the CdSiP₂crystal includes an input surface capable of receiving the at least oneincident beam of electromagnetic radiation and an output surface capableof transmitting an emerging beam of electromagnetic radiation.

In some embodiments, the single crystal of CdSiP₂ is oriented relativeto polarization and propagation directions of the incident radiation soas to allow phase-matched propagation of the incident and emergingradiation. And in certain embodiments, the single crystal of CdSiP₂ hasa total volume of at least 20 mm³.

In various embodiments, the single crystal of CdSiP₂ is free of cracks,twins, voids, inclusions, grain-boundaries, and all other macroscopicdefects throughout all apertures and propagating paths of allelectromagnetic waves interacting therein. In certain embodiments all ofthe incident beams of electromagnetic radiation have wavelengths thatfall in the range 0.5 μm to 10 μm, and in other embodiments theelectromagnetic radiation emerging from said crystal includes at leastone output wavelength that falls in the range 0.5 μm to 10 μm. Someembodiments are configured for transmitting an emerging beam having anaverage power of at least 10 Watts.

Various embodiments further include an input coupler configured toenable the incident beams of electromagnetic radiation to be directedinto the nonlinear optical crystal, and an output coupler configured toenable extraction of at least one emerging beam of electromagneticradiation from the nonlinear optical crystal.

In some embodiments, the output coupler includes an optical filterconfigured to isolate the emerging beam from other beams ofelectromagnetic radiation. Certain embodiments further include anorientation adjustment mechanism configured to adjust an orientation ofthe nonlinear optical crystal relative to directions of propagation andpolarization of the incident beam or beams. Other embodiments furtherinclude a temperature adjusting mechanism configured to adjust atemperature of the nonlinear optical crystal.

In certain embodiments the input coupler includes an input mirror thatis at least partially transmissive of the incident beam or beams, andthe output coupler includes an output mirror that is at least partiallytransmissive of the emerging beam.

In some embodiments the CdSiP₂ single crystal is grown from a compoundproduced by a two-temperature method comprising vacuum sealing Cd, Si,and P in an ampoule in a molar ratio of nominally 1:1:2, not includingany excess quantities added so as to account for a vapor phase above amelt, the Cd and Si being physically mixed at a first end of the ampouleand the P being located at a second end of the ampoule, and heating theCd and Si at least near the melting point of the compound CdSiP₂ whileheating the P to a lower temperature sufficient to cause the P tovolatilize and react with the molten Cd and Si, thereby forming CdSiP₂compound. And in some embodiments the CdSiP₂ single crystal is grown bydirectional solidification from a stoichiometric melt containing Cd, Si,and P in a molar ratio of nominally 1:1:2.

In various embodiments the nonlinear optical device is configured forfrequency conversion by means of phase-matched sum frequency generation,second harmonic generation, phase-matched difference frequencygeneration, phase-matched optical parametric generation, phase-matchedoptical parametric amplification, and/or phase-matched opticalparametric oscillation.

In some embodiments, the electromagnetic radiation emerging from theCdSiP₂ single crystal includes at least one output wavelength ofsubstantially about 6.45 μm. In certain embodiments the nonlinearoptical device is configured for non-critical phase matching and theelectromagnetic radiation emerging from said crystal is temperaturetuned so as to include a wavelength of substantially about 6.45 μm. Andin various embodiments the nonlinear optical device is pumped by asolid-state laser having an output wavelength of 1064 nm.

Another general aspect of the present invention is an infrared lasersurgery scalpel. The infrared laser surgery scalpel includes a singlecrystal of CdSiP₂ having NLO properties, into which at least oneincident beam of electromagnetic radiation can be directed so as togenerate electromagnetic radiation emerging from the crystal thatincludes an output wavelength of substantially about 6.45 μm. In someembodiments the at least one incident beam is a single incident beamproduced by a solid-state laser having a wavelength of substantiallyabout 1064 nm. And in certain embodiments the nonlinear optical deviceis configured for non-critical phase matching and the electromagneticradiation emerging from the crystal is temperature tuned so as toinclude a wavelength of substantially about 6.45 μm.

The features and advantages described herein are not all-inclusive and,in particular, many additional features and advantages will be apparentto one of ordinary skill in the art in view of the drawings,specification, and claims. Moreover, it should be noted that thelanguage used in the specification has been principally selected forreadability and instructional purposes, and not to limit the scope ofthe inventive subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a flow diagram that illustrates the method of the presentinvention for producing CdSiP₂ crystals suitable for use in NLO devices;

FIG. 1B is a sectional side view showing the configuration of a boat andampoule used for crystal growth in an embodiment of the presentinvention;

FIG. 1C is a sectional side view of a transparent furnace used forcrystal growth in an embodiment of the present invention;

FIG. 2 is a graph of the type I phase matching curves for CdSiP₂crystals of the present invention pumped at laser wavelengths of 2055 nm(1), 1550 nm (2), and 1064 nm (3,4);

FIG. 3 is a functional diagram of an embodiment of the NLO device of thepresent invention; and

FIG. 4 is a plot of absorption coefficient vs. wavelength comparing thetransparency range of CdSiP₂ crystals of the present invention with thatof ZnGeP₂, another nonlinear optical crystal of the prior art,illustrating the much lower losses of the CdSiP₂ crystals in the 0.5-2μm wavelength range.

DETAILED DESCRIPTION

With reference to FIG. 1A, the method of the present invention forproducing a single crystal of CdSiP₂ suitable for use in NLO devicesincludes the following steps:

a) placing red phosphorus (P) at one end of a quartz ampoule 100 and aPBN or PBN-coated graphite boat containing cadmium (Cd) and silicon (Si)at the other end 102 of the ampoule in a molar ratio of Cd:Si:P=1:1:2,and evacuating and sealing the ampoule 104;

b) heating the sealed ampoule in a two-zone furnace so that the P end ofthe furnace is maintained at a cold zone temperature of 485° C. and theCd and Si end of the furnace is maintained at a hot zone temperature of1180° C. 106 and waiting until the P fully transports and reacts with Cdand Si to form CdSiP₂ (4-24 hours) 108 and then cooling to roomtemperature 110;

c) loading the resulting synthesized CdSiP₂ ingot into a PBN orPBN-coated graphite boat fitted with a well containing a seed crystal112, and vacuum-encapsulating the boat in a fused silica ampoule 114;

d) heating the encapsulated PBN or PBN-coated graphite boat in a furnacewith an axial gradient of 1-5° C./cm to about 1130° C. and adjusting thetemperature so as to fully melt the CdSiP₂ ingot and partially melt theseed crystal 116;

e) cooling the encapsulated PBN or PBN-coated graphite boat at a rate ofbetween 0.02° C. per hour and 1° C. per hour so as to achievedirectional solidification of the CdSiP₂ at a crystal growth rate of 0.2to 2 mm/hour 118; and

f) cooling the resultant CdSiP₂ crystal to room temperature at a rate ofbetween 5° C./hour and 100° C./hour 120.

FIG. 1B illustrates a PBN-coated graphite boat 10 of a preferredembodiment, the boat being fitted with a well 14 containing a seedcrystal 13, and being vacuum-encapsulated within a fused silica ampoule19. A single crystal seed 13, which should be free of cracks, twins, andsecondary grains, is placed in the seed well 14 of the boat 10. The seedcrystal 13 can be of random orientation, or can be oriented along a highsymmetry direction of the crystal such as [100], [001], [110], [112],etc. In various embodiments, the seed crystal is oriented along thephase-matching direction required for a specific laser application.

FIG. 1C illustrates a dual zone, horizontal temperature gradient orgradient freeze furnace 20 used in an embodiment of the presentinvention to heat the encapsulated graphite boat with an axial gradientof 1-5° C./cm to about 1130° C. and adjust the temperature so as tofully melt the CdSiP₂ ingot and partially melt the seed crystal. Furnace20 generally consists of four concentric cylinders, including an innercylinder 21 known as the muffle tube, a two-piece heater support tube22, 23 a shield tube 24, and an outer gold-coated tube 25 known as themirror tube. Cylinders 21-25 are closed and supported by ceramic endcaps 26, 27. The muffle tube 21 supports the ampoule 19 near theconcentric radial center of all the cylinders 21-25. With ampoule 19located within cylinder 21, the ends thereof are closed with insulation29. Heating coils 31, 32 are helically wound onto heater support tubes22 and 23 and create separate heating zones 33, 34 in their respectivehalves. The heating coil spacing is selected to yield a desired range oflongitudinal temperature gradients.

The method described above has been used to produce single crystals ofCdSiP₂ suitable for use in NLO devices with diameters up to 19 mm ormore and lengths up to several centimeters. These crystals of thepresent invention have been used to measure the following NLOproperties:

1. Wide Wavelength Transparency Range:

The inventors of the present invention have examined the transmissioncharacteristics of CdSiP₂ single crystals produced using the methoddisclosed above, and have found that the CdSiP₂ single crystals aretransparent in the wavelength range of 500 nm to 9500 nm. The measuredtransparency range of a CdSiP₂ crystal of the present invention, plottedas absorption coefficient versus wavelength, is shown in FIG. 4. Thethick line 401, 403 indicates the measured absorption spectrum forCdSiP₂, whereas the thin line 402, 404 indicates the measured absorptionspectrum for a nonlinear optical crystal of the prior art, ZnGeP₂. Notethat the absorption losses between the band edge and 2 μm 401 aresubstantially lower in a CdSiP₂ crystal of the present invention than inthe prior art ZnGeP₂ crystal 402, allowing CdSiP₂ crystals to be pumpedby 1 μm and 1.55 μm lasers, which are processes not phase-matchableusing the prior art crystal ZnGeP₂.

In addition, CdSiP₂ crystals of the present invention exhibit a 2 μmabsorption coefficient up to 20 times lower than that ZnGeP₂, makingCdSiP₂ a promising alternative to ZnGeP₂ for powerful and efficientmid-infrared OPOs pumped by Tm and Ho lasers. The onset of multi-phononabsorption in a CdSiP₂ crystal of the present invention, with peaks at 7μm and 7.6 μm 403, occurs at shorter wavelengths than in the prior artZnGeP₂ crystal, which has a peak at 9 μm 404. This limits the usefulnessof CdSiP₂ crystals for frequency conversion processes involvingwavelengths beyond 6.7 μm.

2. Large Nonlinear Coefficient:

The inventors have measured the nonlinear coefficient of a CdSiP₂ singlecrystal produced using the method disclosed above, and have obtained avalue of 84.5 pm/V, which is 2.56 times that of AgGaSe₂ (33 pm/V) and 7times larger than that of AgGaS₂ (12 pm/V). This was an unexpectedlyhigh value for the nonlinear optical coefficient, since plotting thenonlinear coefficients of other II-VI-V₂ chalcopyrites as a function ofband gap predicted a value closer to 53 pm/V. This measured value of84.5 pm/V is, in fact, the highest nonlinear optical coefficient of anyknown phase-matchable nonlinear optical crystal that can be pumped by asolid state laser.

3. High Thermal Conductivity:

The inventors have measured the thermal conductivity of an 8 mm×8mm×1.97 mm CdSiP₂ sample using a NETZSCH LFA 447 Nanoflash instrument.The sample faces were coated with gold (˜0.1 μm) and carbon paint (˜5μm) to block any transmitted light. Room-temperature (25° C.)measurements of thermal diffusivity, heat capacity, and thermalconductivity yielded values of 7.69 mm²/s, 0.446 J/g/K, and 13.6 W/mKrespectively. This thermal conductivity value is an order of magnitudehigher than that of the prior art NLO crystals AgGaS₂ (1.4 W/mK) andAgGaSe₂ (1.0 W/mK) used in the prior art for shifting 1.06-μm and1.55-μlm lasers into the mid-IR. This high thermal conductivity, coupledwith the low absorption losses illustrated in FIG. 4, enables thegeneration of mid-infrared laser radiation in excess of 10 W.

4. Refractive Indices and Phase Matching Conditions:

Employing the method of prism minimum deviation, the inventors of thepresent invention also measured the principal refractive indices ofCdSiP₂. Using the least square fitting method, the Sellmeier equationswere obtained as follows:

$\begin{matrix}{{n_{o}^{2} = {2.931 + \frac{6.4248*\lambda^{2}}{\lambda^{2} - 0.10452} - {0.0034888*\lambda^{2}}}}{and}} & (1) \\{n_{e}^{2} = {3.4975 + \frac{5.5451*\lambda^{2}}{\lambda^{2} - 0.11609} - {0.0034264*\lambda^{2}}}} & (2)\end{matrix}$

The refractive index data revealed that CdSiP₂ is a negative uniaxialcrystal (n_(e)−n_(o)<0) with a birefringence in the mid-infrared around−0.05). The Sellmeier equations above were used to calculate the phasematching conditions for CdSiP₂ crystals pumped at various laserwavelengths.

FIG. 2 shows the Type I phase matching curves for CdSiP₂ crystals pumpedat laser wavelengths of 2055 nm (201), 1550 nm (202), and 1064 nm (203,204). While curves are presented for these pumping wavelengths asexamples and as indicative of embodiments of the present invention, itwill be understood that the invention is not limited to lasers operatingwith the pumping wavelengths illustrated in FIG. 2, nor is the presentinvention limited to type I interactions. At each phase matching angle,two output wavelengths are generated, an output with a wavelength thatis less than or equal to twice the pump wavelength, termed the “signal”wave, and an output with a wavelength greater than or equal to twice thepump wavelength, termed the “idler” wave.

According to FIG. 2, pumping a CdSiP₂ NLO device with a 2055 nm laser(curve 201) at internal phase matching angles between 43 and 45 degreesrelative to the c-axis (optic axis) will generate output wavelengthsbetween 2.6 μm and 9.31 μm. Pumping with a 1550 nm laser (curve 202) atinternal phase matching angles between 45.5 and 60.5 degrees relative tothe c-axis (optic axis) will generate output wavelengths from 1.71 μm togreater than 10.5 μm. And pumping with a 1064 nm laser at internal phasematching angles between 50 and 90 degrees relative to the c-axis (opticaxis) will generate signal output wavelengths from 1.1 μm to 1.3 μm(curve 203) and idler output wavelengths from 6 μm to greater than 10.5μm (curve 204). Although 1064 nm pumping is unable to access the 1.3 μmto 6 μm range, CdSiP₂ does allow non-critically phase matched output(i.e. 90-degree phase-matching with no walk-off) around 6.2 μm, whichwould be advantageous for medical applications in this range.

5. Temperature-Tunable Devices

The present inventors also measured the change in the ordinary andextra-ordinary refractive indices, and the consequent change in thecorresponding Sellmeier equations, as a function of temperature. Theresulting temperature-dependent Sellmeier coefficients are listed belowin Table 1, and indicate that the CdSiP₂ crystals of the presentinvention are more temperature-tunable than most other nonlinear opticalchalcopyrite crystals known in the prior art. The data indicate, forexample, that a 2-μm pumped OPO based on CdSiP₂ can be tuned inwavelength between 3.4 and 4.8 μm by varying the temperature of theCdSiP₂ crystal between 10° C. and 70° C. Likewise, a 1.064-μm-pumpedType I CdSiP₂ OPO will produce non-critically phase-matched output at6.18 μm at 25° C., but can be temperature tuned to producenon-critically phase-matched output at 6.45 μm at 99° C. (a modesttemperature increase that can easily be achieved in practical hardware).The ability to efficiently directly generate 6.45 μm radiation from a1.064 μm-pumped non-critically phase-matched OPO is valuable for lasersurgery applications.

TABLE 1 n_(o) n_(e) A 3.0449 + 1.214 × 10⁻⁴T(K) 3.3978 + 1.224 ×10⁻⁴T(K) B 6.1164 + 5.459 × 10⁻⁴T 5.4297 + 6.174 × 10⁻⁴T(K) C 0.00348880.0034264 D 0.10452 0.11609

-   -   CdSiP₂ o- and e-polarized temperature-dependent Sellmeier        coefficients, where n²=A+Bλ²/(λ²−D)−Cλ².

FIG. 3 is a functional illustration of an embodiment of an NLO devicemade of a single crystal of CdSiP₂ of the present invention. The inputor pump wave 305 is incident on the CdSiP₂ NLO crystal 306 and generatessignal 307 and idler 308 output waves. The crystal is cut so that wave305 entering the crystal 306 will propagate through the crystal 306 at aphase matching angle 309 relative to the c-axis 310. The wavelengths ofthe signal wave 307 and idler wave 308 can be tuned by varying the phasematching angle 309 by rotating the crystal 6 about the tuning axis 311.This conversion process is called optical parametric generation (“OPG”)if no mirrors are used to resonate the output wavelengths, and is calledoptical parametric oscillation (“OPO”) if the embodiment includes aninput mirror 312 which is highly transmissive at the pump wavelength andhighly reflective at the signal wavelength and/or the idler wavelength,and an output coupler 313 which is highly transmissive at the pumpwavelength and partially reflective at the signal wavelength and/or theidler wavelength. A crystal heater 314 can be used to temperature-tunethe output wavelength of the nonlinear optical device, particularly inthe case of a non-critically phase-matched OPO. Other NLO conversionprocesses such as difference frequency generation (“DFG”), sum frequencygeneration (“SFG”), and second harmonic generation (“SHG”) can also beperformed with CdSiP₂ crystals of the present invention.

In various embodiments the polarizations of the interacting waves andthe crystallographic orientation of the optic axis 310 and tuning axis311 are chosen so as to satisfy the conditions for a given phasematching type (type I, type II, etc.) and so as to maximize theefficiency of the frequency conversion process using the principlesdetailed for example in M. J. Weber (ed.) CRC Handbook of Laser Scienceand Technology, Vol. 3., (CRC Press, Boca Raton, Fla., 1986), relevantportions incorporated by reference for all purposes.

One example of a device according to FIG. 3 is a medical laser scalpel.The device is pumped by a conventional laser such as a Nd:YAG laser at1.06 μm and produces an output laser beam of approximately 6.45 μm,which is a wavelength with high soft tissue absorbance and cuttingefficiency. In various embodiments of this device, the CdSiP₂ crystal306 is oriented for noncritical phase matching with no “walk-off” of theemerging beam relative to the incident beam. The crystal 306 is heatedby the heater 314 to a temperature of approximately 100° C., so as totune the output wavelength to approximately 6.45 μm. A specific medicallaser scalpel embodiment is discussed in more detail in Example 3 below.

It can be seen from the discussion above that the claimed singlecrystals of CdSiP₂ are novel NLO crystals having excellent NLOproperties. The claimed NLO devices made using the claimed CdSiP₂crystals have superior properties as compared to prior art NLO devicesmade using either AgGaSe₂ or AgGaSe₂ crystals. In particular, theclaimed devices have a higher nonlinear coefficient and higher thermalconductivity than these prior art devices, making them more attractivefor high average power laser frequency conversion in the mid-infrared.

Example 1

22.0453 g of red phosphorus was loaded into one end of a heavy-walled,fused silica ampoule, and a PBN-coated graphite boat containing 9.9948 gsilicon and 40.0034 g cadmium was loaded into the opposite end. Theampoule was evacuated, sealed, and loaded into a two-zone shunt-typetube furnace with a Hastelloy C-276 protective liner. The programmedheating cycle was as follows: zone 1 (hot zone, containing the boat withCd and Si) was heated at 50° C./hr to 1025° C., 37.9-hr soak, 25° C./hrto 1180° C., 8-hr soak, cool at 100° C./hr; zone 2 (containing P) washeated at 50° C./hr to 485° C., 28-hr soak, 25° C./hr to 1180° C., 8-hrsoak, cool at 100° C./hr.

The resulting single phase polycrystalline ingot was loaded into aPBN-coated graphite boat with a seed well containing a CdSiP₂ singlecrystal seed oriented for horizontal growth along the (001) c-axisdirection with the (110) axis normal to the top surface. The boat,charge, and seed were vacuum encapsulated in a heavy-walled, fusedsilica ampoule and heated in a two-zone transparent furnace at 16.7°C./hour to set points of 1125° C. and 1115° C. respectively to establishan axial temperature gradient of 1.5° C./cm. The set points weregradually raised to 1130° C. and 1120° C. to partially melt the seedcrystal, followed by cooling at 0.15° C./hr for 160 hours to inducedirectional solidification at a rate of 1 mm/hour. Once fullysolidified, the crystal was cooled at 5° C./hour to room temperature.The resulting “D”-shaped crystal was 19 mm wide, 10 mm high and 140 mmlong: the first 15 mm of growth were a clear, crack-free, single crystalgrain that reproduced the orientation of the seed crystal.

Example 2

A CdSiP₂ crystal obtained by using the procedure of Example 1 withexcellent mechanical properties was cut and polished into a body of3×6×12 mm³ after determination of the crystallographic axes a and c atphase matching angles of θ_(m)=42.5 degrees and φ=45 degrees. Thecrystal was placed in the optical path shown in FIG. 3 without mirrors312 and 313. The incident wave 305 was from a frequency-doubled,Q-switched CO₂ laser of wavelength 4.64 microns. The emerging secondharmonic wave 307, 308 with a wavelength of 2.32 microns was obtained.The uncoated crystal achieved 30% optical-to-optical conversionefficiency.

Example 3

A CdSiP₂ crystal obtained by using the procedure of Example 1 was cutinto a body of 6×6.75×9.5 mm³ after determination of thecrystallographic axes a and c at phase matching angles of θ_(m)=90degrees and φ=45 degrees. The residual losses measured for the relevantpolarizations (e for the pump and o for the signal and idler) were 0.185cm⁻¹ at 1064 nm, 0.114 cm⁻¹ near 1.3 μm, and 0.014 cm⁻¹ near 6.4 μm.Both faces were AR-coated for the three wavelengths (pump, signal, andidler) and the 8-layer coating resulted in averaged reflectivity persurface of ˜0.35% at 1064 nm, ˜0.4% at 1275 nm and ˜0.8% at 6400 nm. Thecrystal was placed in the optical path shown in FIG. 3. The incidentwave 305 was from a Q-switched 1064 nm Nd:YAG laser: no two-photonabsorption was observed. The internal crystal angle was approximately0.67° below the desired 90° interaction angle, resulting in a signalwavelength 307 of 1.277 μm and an idler wavelength 308 of 6.398 μm,which was increased to 6.45 μm by heating the crystal 306 using a heater314. This interaction is extremely useful for laser surgery applicationsat 6.45 μm, including minimally-invasive neurosurgery. An extremelyeffective laser scalpel for cutting soft tissue has been demonstrated atthis wavelength using a free electron laser (see Nature 371, 416-419, 29Sep. 1994, incorporated herein in its entirety for all purposes), whichis an enormous and extremely expensive laser source. The CdSiP₂ crystalof the present invention will allow direct, efficient, non-criticallyphase-matched generation of the desired wavelength from a compact,powerful, and readily available Nd:YAG pump laser.

The foregoing description of the embodiments of the invention has beenpresented for the purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed. Many modifications and variations are possible in light ofthis disclosure. It is intended that the scope of the invention belimited not by this detailed description, but rather by the claimsappended hereto.

What is claimed is:
 1. A nonlinear optical device comprising a negativeuniaxial II-VI-V₂ crystal belonging to the space point group 42m andhaving NLO properties, whereby at least one incident beam ofelectromagnetic radiation can be directed into said crystal so as togenerate electromagnetic radiation emerging from said crystal thatincludes at least one output wavelength different from the wavelengthsof all incident beams of radiation, and wherein said crystal is a singlecrystal of CdSiP₂.
 2. The nonlinear optical device of claim 1, whereinthe single crystal of CdSiP₂ is oriented relative to polarization andpropagation directions of the incident radiation so as to allowphase-matched propagation of the incident and emerging radiation.
 3. Thenonlinear optical device of claim 1, wherein the single crystal ofCdSiP₂ has a total volume of at least 20 mm³.
 4. The nonlinear opticaldevice of claim 1, wherein the single crystal of CdSiP₂ is free ofcracks, twins, voids, inclusions, grain-boundaries, and all othermacroscopic defects throughout all apertures and propagating paths ofall electromagnetic waves interacting therein.
 5. The nonlinear opticaldevice of claim 1, wherein all of the incident beams of electromagneticradiation have wavelengths that fall in the range 0.5 μm to 10 μm. 6.The nonlinear optical device of claim 1, wherein the electromagneticradiation emerging from said crystal includes at least one outputwavelength that falls in the range 0.5 μm to 10 μm.
 7. The nonlinearoptical device of claim 1, wherein the CdSiP₂ single crystal is grownfrom a compound produced by a two-temperature method comprising: vacuumsealing Cd, Si, and P in an ampoule in a molar ratio of nominally 1:1:2,not including any excess quantities added so as to account for a vaporphase above a melt, the Cd and Si being physically mixed at a first endof the ampoule and the P being located at a second end of the ampoule;and heating the Cd and Si at least near the melting point of thecompound CdSiP₂ while heating the P to a lower temperature sufficient tocause the P to volatilize and react with the molten Cd and Si, therebyforming CdSiP₂ compound.
 8. The nonlinear optical device of claim 1,wherein the CdSiP₂ single crystal is grown by directional solidificationfrom a stoichiometric melt containing Cd, Si, and P in a molar ratio ofnominally 1:1:2.
 9. The nonlinear optical device of claim 1, wherein thenonlinear optical device is configured for frequency conversion by meansof phase-matched sum frequency generation.
 10. The nonlinear opticaldevice of claim 1, wherein the nonlinear optical device is configuredfor frequency conversion by means of second harmonic generation.
 11. Thenonlinear optical device of claim 1, wherein the nonlinear opticaldevice is configured for frequency conversion by means of phase-matcheddifference frequency generation.
 12. The nonlinear optical device ofclaim 1, wherein the nonlinear optical device is configured forfrequency conversion by means of phase-matched optical parametricgeneration.
 13. The nonlinear optical device of claim 1, wherein thenonlinear optical device is configured for frequency conversion by meansof phase-matched optical parametric amplification.
 14. The nonlinearoptical device of claim 1, wherein the nonlinear optical device isconfigured for frequency conversion by means of phase-matched opticalparametric oscillation.
 15. The nonlinear optical device of claim 1,wherein the electromagnetic radiation emerging from said crystalincludes at least one output wavelength of substantially about 6.45 μm.16. The nonlinear optical device of claim 1, wherein the nonlinearoptical device is configured for non-critical phase matching and theelectromagnetic radiation emerging from said crystal is temperaturetuned so as to include a wavelength of substantially about 6.45 μm. 17.The nonlinear optical device of claim 1, wherein the nonlinear opticaldevice is pumped by a solid-state laser having an output wavelength of1064 nm.
 18. An infrared laser surgery scalpel, comprising a singlecrystal of CdSiP₂ having NLO properties, into which at least oneincident beam of electromagnetic radiation can be directed so as togenerate electromagnetic radiation emerging from said crystal thatincludes an output wavelength of substantially about 6.45 μm.
 19. Thenonlinear optical device of claim 18, wherein the at least one incidentbeam is a single incident beam produced by a solid-state laser having awavelength of substantially about 1064 nm.
 20. The nonlinear opticaldevice of claim 18, wherein the nonlinear optical device is configuredfor non-critical phase matching and the electromagnetic radiationemerging from said crystal is temperature tuned so as to include awavelength of substantially about 6.45 μm.